The invention relates to active acoustic attenuation systems, and more particularly to a system for a correlated input acoustic wave, i.e. periodic, band limited, or otherwise having some predictability.
The invention arose during continuing development efforts directed toward the subject matter shown and described in U.S. Pat. Nos. 4,677,676, 4,677,677, 4,736,431, 4,815,139, 4,837,834, 4,987,598, 5,022,082, and 5,033,082, incorporated herein by reference.
Active acoustic attenuation involves injecting a canceling acoustic wave to destructively interfere with and cancel an input acoustic wave. In an active acoustic attenuation system, the output acoustic wave is sensed with an error transducer, such as a microphone or an accelerometer, which supplies an error signal to an adaptive filter control model which in turn supplies a correction signal to a canceling output transducer, such as a loudspeaker or a shaker, which injects an acoustic wave to destructively interfere with the input acoustic wave and cancel same such that the output acoustic wave at the error transducer is zero or some other desired value.
The present invention provides an active acoustic attenuation system .for attenuating correlated acoustic fields, including sound and vibration, and eliminates the need for an input transducer, such as an input microphone or an accelerometer sensing the input acoustic wave. It is known in the prior art that the input signal may be provided by one or more error signals, in the case of a periodic noise source, "Active Adaptive Sound Control In A Duct: A Computer Simulation", J. C. Burgess, Journal of Acoustic Society of America, 70(3), September 1981, pages 715-726. It is known in the prior art to sum the error signal with the model output correction signal, and to supply the resultant sum as the model input. The present invention eliminates the need to use the model output correction signal in the sum supplied to the model input.
In the prior art, the input signal is processed through the model M and then through a speaker-error path SE copy, and then is summed with the error signal to provide the noted resultant sum supplied as the model input. The present invention recognizes that, for purposes of summing with the error signal, the processing of the input signal through M and then through SE copy may be reversed. In the present invention, the input signal is processed through the model M in normal manner. The input signal is also processed in a parallel path initially through an SE copy and then through a model M copy, and then is summed with the error signal to provide a resultant sum which is supplied as the model input. In the present invention, SE copy and M copy are in series between the model input and the error signal summer, and the input signal is processed first through SE copy and then through M copy. This is in contrast to the prior art wherein M and SE copy are in series between the model input and the error signal summer, and the input signal is processed first through M and then through SE copy for summing with the error signal.